With the rapid development of liquid crystal display technology, liquid crystal displays have been widely applied in display apparatuses such as notebooks, monitors and TVs. Thin film transistor liquid crystal display (TFT-LCD) is one of the numerous liquid crystal displays. Thin film transistor liquid crystal display is a display in which each liquid crystal pixel point in the liquid crystal panel is driven by a thin film transistor (TFT) integrated behind it such that it's possible to display screen information with high speed, high brightness and high contrast.
In a conventional TFT liquid crystal panel, a connection circuit structure for thin film transistors is shown in FIG. 1. Each pixel structure of the liquid crystal panel includes a data line 1, a gate line 2 and a pixel electrode 3. A thin film transistor 9 is formed at an intersection of the data line 1 and the gate line 2. The thin film transistor 9 has a gate connected to the gate line 2, a source connected to the data line 1, and a drain connected to the pixel electrode 3. An equivalent circuit of the above-mentioned circuit structure is shown in FIG. 2, wherein 11 is a liquid crystal capacitance Clc, and 12 is a storage capacitance Cst.
When the thin film transistor 9 is reversely biased, due to the presence of free electrons attached at the active layer, a tiny current flows through the drain and the source, which is called OFF state current Ioff, also commonly known as leakage current of TFT pixel structure. When designing the TFT pixel structure, an excessive large OFF state current would cause some problems. For example, if the OFF state current is too large, the pixel voltage sustaining characteristic will be affected to a certain extent, that is, the sustained voltage drops too fast when the gate is off to cause a changes over 2 gray-scales recognizable for the human eye, which would cause flicking. Further, if the OFF state current is too large, residual DC component increases when the pixel is discharged, causing charge residual and an afterimage.